I have a classifieds website, and on each classified there is a tip-form where users may tip a friend about the classified. The tip-forms' action is set to a php-page, which mails the email after sanitizing etc...

I have to filter away spam etc so that my email-server don't get blacklisted or anything...

I have my own server (VPS, Linux) and have thought about a solution...

How does this sound to you:

   1- Install a mail-server
   2- Configure Firewall to ONLY allow connections to the mail-server from my website
   3- Configure the mail-server so that a maximum of 'x' emails may be sent every 5 minutes or so
   4- Create a php filter before sending the mail, which checks for 'bad' words.
   5- If necessary, as last resort, ask the user a question (ex 5+5) before submitting form

I would rather preferr if I didn't have to implement the 5th implementation above...

What do you think?

Also, another q I have that you may answer is:

If an email-server gets blacklisted, is there any way to un-blacklist it? Or whats the solution if this happens?

Thanks